This invention relates to circuits for digitally processing exposure control information for cameras and more particularly to those for digitally processing scene brightness or luminance level information.
As is generally known, there exists among the exposure control informations including the shutter speed or the exposure time T, the aperture A, the film sensitivity S and scene brightness B a relationship expressed by the following equation: EQU B=K.multidot.A/(T.multidot.S) (1)
where K is a constant. In obtaining an optimum exposure by computation based upon the above equation, the respective informations given are usually logarithmically compressed so that they may conveniently be dealt with as exponential informations, particularly because the scene brightness B varies over very wide range with the ratio of its maximum to its minimum reaching 10.sup.6 or greater. Logarithmic compression of the exposure informations enables computation to be effected simply by addition and subtraction, and makes it possible to employ electric circuitry of very simple structure for such computation. Obviously, the exposure control information obtained in this manner is finally logarithmically expanded so as to be utilized as an actual control information.
Logarithmic compression of the scene brightness information has conventionally been effected on an analog basis by use of diode means having a logarithmic characteristic. This makes the circuit construction very simple, but at the same time involves serious deficiencies that the diode characteristic must be compensated for over the wide range of use of the diode and that there occurs a problem of thermal stability because of the temperature characteristic inherent to the diode element.
Further, for digitalization of an electronic shutter circuit, the analog-digital conversion circuit employed to digitalize the light quantity must have a diode provided in its initial stage for logarithmic compression of the analog quantity. This is because the scene brightness information varies over so wide a range, as pointed out above, that any direct digital conversion of such information may take excessive time, and hence the extended waiting time for shutter operation may result in missing the desired picture. On the other hand, the greatest advantage of digitalization is that it alleviates the exacting requirements for stability against variations in temperature and source voltage as well as for part characteristics. Such advantage of digitalization must be half lost in cases where diode means, which is required itself to meet strict conditions for thermal stability and operation characteristics, is used for logarithmic compression in the circuit for analog-digital conversion of the quantity of light measured.